The operation was a sanctions-enforcement action, not a naval combat incident. The Ministry of Defence said Royal Marine Commandos and National Crime Agency officers boarded the vessel in the early hours, supported by aircraft from the Maritime Air Group, an RAF P-8 aircraft, HMS Sutherland and HMS Ledbury. The government said the operation lasted six hours and that the vessel would be held and monitored off England's south coast while investigations continued.

The legal status of the ship is central to the story. The UK sanctions-list entry for RUS3133 identifies the vessel as MYRTOS, also known as Smyrtos, with IMO number 9389100, and says it is, has been, or is likely to be involved in activity whose object or effect is destabilising Ukraine or undermining or threatening Ukraine's territorial integrity, sovereignty or independence.

Table: Smyrtos sanctions and interdiction timeline

DateStepSource
Oct. 15, 2025UK designation RUS3133 listed MYRTOS / Smyrtos, IMO 9389100, as a sanctioned ship.UK Sanctions List
March 25, 2026The UK said armed forces and law-enforcement officers would be able to interdict sanctioned shadow-fleet vessels transiting UK waters.GOV.UK
June 14, 2026Royal Marines and National Crime Agency officers boarded Smyrtos in the Channel.UK Ministry of Defence

Source: UK Ministry of Defence, UK Sanctions List, AP.

The government had prepared the policy route in March. A Downing Street and Ministry of Defence statement then said British military personnel would be able to board sanctioned ships transiting UK waters as part of a wider effort with Joint Expeditionary Force partners to restrict Russia's use of tankers to move oil despite sanctions.

The Sunday statement said Prime Minister Keir Starmer approved the March policy and said the June 14 enforcement action took place in international waters in accordance with domestic and international law. It did not publish the boarding warrant, the evidential threshold used by officers, or the detailed cargo status of Smyrtos.